1. Field of the Invention
This invention relates to an image signal processing apparatus. More particularly, the present invention relates to an image signal processing apparatus having an image pickup device and a color filter. The invention can be applied, for example, to a camera for capturing an image for still (as opposed to moving) display but is not limited thereto.
2. Related Background of Art
It is known to use a frame transfer type CCD as an image pick-up device, with a stripe color filter as shown in FIG. 4. Other types of filter, such as mosaic types, in which a column of filter portions is not all the same color, can provide improved resolution. FIG. 5 shows an example of a mosaic type filter, and FIGS. 2 and 3 show Bayer-type mosaic filters, known from US Pat. No. 3,971,065. The Bayer-type filter has Y portions which pass a luminance component of an image, arranged in an offset manner which can enable a high resolution to be achieved, e.g. by interpolation from adjacent lines of Y signal. However, mosaic types of filters tend not to work well with a frame transfer type CCD. Under some circumstances there is the possibility with a frame transfer type CCD that there may be some contamination of the charge amount for one pixel by the charge amount for an adjacent pixel in the column direction. Provided that all the pixels in a column represent the same color, this vertical `smearing` of the read-out signal results in a slight loss of vertical resolution. However, if the pixels represent different colors, so that a charge amount for one color is contaminated by a charge amount for a different color, false color reproduction might result, and this would be less acceptable than a loss of resolution. This problem arises equally with a camera for a still picture and a camera for a moving picture.
Mosaic type filters can be used with other types of image pick-up devices. Some of these, such as an interline-type CCD, can also provide an interlace output signal. Such an interlace output can be advantageous. TV type display devices normally require an interlace type input signal. A frame transfer type CCD outputs its entire image signal in a single destructive read operation. Typically this will provide one field of video signal. For a moving image display, the next field signal can be obtained by exposing the image pick-up device again and reading out the signal again, so that successive fields of video signal come from successive exposures of the CCD. However, for a still video display this will result in a still picture being reproduced from two field signals derived from exposing the image pick-up device at two different times, and so any object which is moving or changing with time will be blurred or otherwise degraded in reproduction. In practice, a single field signal is normally read and stored from a frame transfer type CCD for still video reproduction, and a second field signal is synthesised from the single field signal, e.g. by interpolating between adjacent lines. However, this tends to result in a lower resolution display than one in which both field signals are derived in an interlace manner from a single exposure of the image pick-up device.
An interline type CCD, for example, can be driven in interlace manner so that in a first field only alternate lines of the image pick-up device are read, and in a second field the intervening lines are read. Both fields can be read from a single exposure of the CCD, allowing an interlace output without movement flicker for a still video camera. However, if a Bayer-type filter of FIG. 2 or FIG. 3 is combined with this type of CCD device, the following problems arise.
As shown in FIG. 6, only pixels Y.sub.1 and Y.sub.2 are used for interpolating the missing luminance signal pixel (shaded in FIG. 6) when reading the second line, because pixels Y.sub.3 and Y.sub.4 are only read in another TV field period.
If the missing luminance signal is interpolated by using color signals, this will cause zigzag noise at the edge portion of a diagonal line on the screen. This is caused because the carriers of color signals affect the luminance signal. This problem does not depend on the type of CCD used.
In the color processing, the problem arises that each field signal can contain only one type of color. For example in FIG. 2 the first field signal will contain only the cyan color signal and the second field signal will contain only the yellow color signal.
Consequently the interline type of CCD device is not easily used with a Bayer-type filter.
The latter problem will also arise when using the filter of FIG. 5. It can be avoided by using the filter of FIG. 9. With this type of filter the color image signals produced by the CCD device are converted into Y, R, B signals by signal processing. These signals may then be compressed by sub-sampling using the patterns as shown in FIGS. 10(a)-10(c).
However, in this case the image signal processing apparatus has to include a converting circuit for converting the output signals from the CCD device into a luminance signal and color signals, and further it has to include a sub-sampling circuit if sub-sampled signals are desired. These circuits are obstacles to reducing the size or the cost of the apparatus and for improving the quality of an image on the screen.